Interdigitated capacitors

ABSTRACT

The specification describes matched capacitor pairs that employ interconnect metal in an interdigitated form, and are made with an area efficient configuration. In addition, structural variations between capacitors in the capacitor pair are minimized to provide optimum matching. According to the invention, the capacitor pairs are interdigitated in a manner that ensures that the plates of each pair occupy common area on the substrate. Structural anomalies due to process conditions are compensated in that a given anomaly affects both capacitors in the same way. Two of the capacitor plates, one in each pair, are formed of comb structures, with the fingers of the combs interdigitated. The other plates are formed using one or more plates interleaved between the interdigitated plates.

RELATED APPLICATIONS

This application is a division of application Ser. No. 10/886,763 filedJul. 8, 2004 now U.S. Pat. No. 7,022,581.

FIELD OF THE INVENTION

This invention relates to capacitors for integrated circuits.

BACKGROUND OF THE INVENTION

A simple and cost-effective way of forming capacitors for integratedcircuits is to use side-by-side runners of interconnect metal. Toenhance capacitor area the runners may be interdigitated. See U.S. Pat.No. 6,383,858, issued May 7, 2002, and incorporated herein by reference.Since the capacitor dielectric may be formed by the interleveldielectric of the integrated circuit (IC) these passive devices can beformed with no additional IC processing steps. Capacitance may befurther increased by stacking interdigitated structures in a multi-levelconfiguration. (See the patent referenced above).

Capacitors of this type may be integrated with active devices in aconventional integrated circuit, or may comprise portions of integratedpassive devices (IPDs), where the capacitors are combined withinductors, resistors etc., to form a passive device network. IPDstypically are formed on high resistivity substrates and may have noactive elements. In either case, capacitors are commonly used in pairsor other groupings.

For some IC applications, paired capacitors are required to haveequivalent electrical characteristics. The straightforward method toobtain matched capacitor pairs is to form them with identical structuresusing the same processing steps. While that approach is largelysuccessful, precise matching of capacitors, even when formedside-by-side on a substrate wafer, may not result. This is often due todimensional variations in the runners, or the dielectric. Thesevariations occur in both the thickness dimension, for example, theinterlevel dielectric thickness, and in the lateral dimension, and aretypically attributed to processing conditions. The thickness dimensionis usually thought of as the z-direction, with the lateral dimension(s)corresponding to the x-y plane of the substrate wafer. Capacitor pairmismatch commonly occurs due to localized thickness variations in themetal runners from one capacitor structure to another.

BRIEF STATEMENT OF THE INVENTION

We have developed a new capacitor structure that employs interconnectmetal in an interdigitated form, but provides additional simplificationwhen capacitors are used in pairs. In addition, it significantly reducesstructural variations between capacitors in a capacitor pair. Accordingto the invention, paired capacitors are formed in an interdigitatedconfiguration, in a manner that ensures that the plates of each pairoccupy common area on the substrate. The interdigitated structure islaid-out such that structural anomalies due to process conditions arecompensated in that a given anomaly affects both capacitors in the sameway. Two of the capacitor plates, one plate for each of the matched pairof capacitors, are formed with electrically conductive comb structures,with the fingers of the combs interdigitated. The other plates for eachof the capacitors are formed using one or more conductive runnerstructures interleaved between the interdigitated plates.

BRIEF DESCRIPTION OF THE DRAWING

The invention may be better understood when considered in conjunctionwith the drawing in which:

FIG. 1 is a simple circuit diagram for a capacitor pair of theinvention;

FIG. 2 is a schematic view showing an implementation of the capacitorpair using a prior art approach;

FIG. 3 is a schematic view of a capacitor pair constructed according tothe invention;

FIG. 4 is a schematic diagram of a stacked capacitor using theconfiguration of FIG. 3;

FIGS. 5-8 and 10 are schematic views similar to that of FIG. 3 showingalternative embodiments of the invention; and

FIGS. 9 and 11 are section views of stacked capacitors using the basicconfiguration of FIGS. 9 and 11 respectively.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows one circuit embodiment of the invention that will be usedfor the detailed description. Capacitors 11 and 15 may be connected inthe configuration shown, with I/O connections a, b, and c. Capacitor 11is formed by plates 12 and 13, and capacitor 15 is formed by plates 16and 17. This capacitor pair is shown implemented using an interdigitatedinterconnect structure in FIG. 2. Substrate 21 represents any known ICsubstrate including but not limited to silicon, gallium arsenide, indumphosphide or an IPD substrate such as ceramic. The substrate is showncut-away indicating that this is a portion, typically a small portion,of a larger substrate or wafer.

Due to processing variables, it is not uncommon for the thickness of theblanket layer that is patterned to form the discrete interdigitateddevices 11 and 15 in FIG. 2 to vary over the surface of the substrate.In IC device manufacture, the substrate is typically a silicon waferwith a plurality of identical die formed in a repeated pattern on asurface of the wafer. State of the art wafers may be as large as 12″(300 mm) in diameter. Thousands of IC devices are produced from a singlewafer. In IPD manufacture, the substrate is typically an insulatingsubstrate, for example, ceramic. The device sites for IPDs arecharacteristically larger than for IC die or chips. In each case, anylayer that is deposited over the whole substrate will vary in thicknessin the x-y plane of the substrate or wafer. This thickness variation isindicated by the arrow, and “Δt”, in the y or vertical direction of FIG.2. Similar thickness variations also occur in an orthogonal, x directionof FIG. 2. It means that the layers that form capacitor 11 may have athickness different from the layers that form capacitor 15. In that casethe capacitance of capacitor 11 will be different from the capacitanceof capacitor 15. If the variations indicated by Δt are significant,making a matched pair of capacitors will be difficult or impossible.

An interdigitated capacitor pair with a common interleaved plateaccording to one embodiment of the invention is shown in FIG. 3. Thesubstrate 26 may be an IC wafer, or an IPD substrate, the surface ofwhich typically is covered with an insulating layer, for example SiO₂for a silicon wafer. In this layout the electrode structure 22corresponds approximately to electrode structure, or plate, 12 of FIGS.1 and 2, and electrode structure 27 corresponds approximately toelectrode structure, or plate 17 of FIGS. 1 and 2. Each capacitor plate22 and 27 has a comb structure with the fingers of plate 22interdigitated with the fingers of plate 27. Plates 13 and 16 of FIGS. 1and 2 have been combined as a common interleaved plate 31.Interdigitated structure 22 is connected as node a, interdigitatedstructure 27 is connected as node b, and interleaved structure 31 isconnected as node c.

As shown in FIG. 3, the spacing between edges of interleaved runner cand edges of the fingers of plates 22 and 27 (shown as “x” in FIG. 4)may be less than the spacing between edges of interleaved runner c andedges of the portions of plates 22 and 27 that do not form the fingers(shown as “p” in FIG. 3). Providing a different spacing in the tworegions lessens the capacitance between the portions of interleavedrunner c that provide direction reversals and the non-finger portions ofplates 22 and 27, resulting in better control of the capacitance, and ofthe resulting matched capacitor pair. A recommended, but not required,range for the ratio p/x is 3 to 20.

The capacitor plates 12, 13, 16 and 17, are preferably formed from asingle level of interconnect. It may be any convenient interconnectlevel. The metal may be any suitable conductive material, metal orcompound, including but not limited to Al, Cu, Au, Ni, Ta, Ti, TaN, TiN,WN, Si, Pt, or combinations thereof for example suicides. From a processstandpoint, it is most convenient to form the capacitor plates from thesame metal used for interconnecting or fabricating other devices on theIC. A metal layer is blanket deposited on the substrate, or on any ofthe interlevel dielectric layers of the IC, and patterned byconventional photoresist techniques to form both the deviceinterconnections for that level, and the capacitor plates shown in FIG.3. If desired, and also within the scope of the invention, a separatestep may be used to deposit and pattern a layer to form the capacitorplates. In that case a different metal may be used for the ICinterconnects and the capacitor plates. Also within the scope of theinvention is the use of different metals to form the variousinterdigitated structures of the capacitor. This option is most likelyto be used in multi-level structures, such as those described in moredetail below.

The interdigitated metal pattern shown in FIG. 3 is covered with adielectric layer that fills the space between the interdigitated metalrunners to form the dielectric between the capacitor plates, thusforming the capacitor pair. The dielectric layer may also serve as theinterlevel dielectric between metal levels. For silicon wafers,preferably the dielectric layer is SiO₂, but other insulating materials,such as silicon nitride, or even a polymer such as polyimide, may beused. One skilled in the art would know what dielectric materials to usefor other than silicon wafers.

Other step sequences may be used to reach the same or similar results.The blanket metal layer may be deposited, as well as a blanketdielectric layer, and the combined layers patterned to form theinterdigitated structure. This may be a more likely sequence inproducing IPDs. In the usual IC manufacturing process, where transistordevices are part of the IC structure, the metal layers are oftenpatterned prior to depositing the interlevel dielectric. However, eitherof these step sequences, as well as others, is useful.

In state-of-the-art integrated circuits with, for example, a 0.13 microndesign rule, the runners are very small and the space betweeninterdigitated runners is small. Continuing the 0.13 micron design ruleexample, interdigitated runners 12, 13, 16 and 17 in FIG. 2 may have awidth of 0.16 to 0.5 microns, a thickness of 0.2 to 1. micron, and aspacing of 0.18 to 0.55 microns.

The efficiency of the layout of the capacitor pair in FIG. 3, whencompared with that of FIG. 2, is evident. Moreover, it is intuitivelyapparent that variations like Δt will have an equivalent influence onboth capacitors. This allows matched capacitor pairs to be produced evenwhen process conditions cause significant variations in Δt.

FIG. 3 as shown may be a single level capacitor, or it may haveadditional levels above or below that shown where added capacitors maybe stacked. A stacked interdigitated device is represented in theschematic of FIG. 4. The levels 1-5 indicate levels of metalization inan IC or IPD device. The first level, the first layer that is formed onthe substrate (not shown), is the bottom level of the finishedstructure. In FIG. 4, the first level corresponds to a section through4-4 of FIG. 3. For clarity, not all of the runners that appear in FIG. 3are depicted in FIG. 4, but the pattern is evident. Each subsequentlevel is formed as shown. Typically each level will be identical to, orsimilar to, the level shown in FIG. 3.

The stacked device embodiment may be used for at least two usefuloptions. The matched capacitor pair at each level may be utilized in anIC or IPD and interconnected with other devices thereon as a separatematched pairs of capacitors, meaning that five separate matched pairs ofcapacitors may be formed in the same die area that is occupied by thedevice of FIG. 4. This option offers the possibility that each matchedpair of capacitors may be different physically than the matched pair ofcapacitors of the other layers such as having a different interdigitatedstructure resulting in different capacitance than the matched pairs ofcapacitors of the other layers. Another option for this structure is tointerconnect the “a” plates from each level, as well as the “b” and “c”plates respectively. When the capacitance of the matched pair ofcapacitors of each level are identical, this results in a matchedcapacitor pair with n times the capacitance of a matched pair ofcapacitors made on a single level. In FIG. 4, n is five as five levelsare illustrated. A convenient method for implementing this embodiment isto provide through hole interconnections between corresponding plates ofadjacent levels. In FIG. 4, interconnections between correspondingplates at each level 1-5 are shown at 41.

For the purpose of illustration only, five metalization levels arerepresented in the IC device depicted in FIG. 4. That suggests that theIC device depicted has at least five-level metalization. Obviously, theIC may have more or fewer levels of metal, all or less than all of themetalization layers may be used to fabricate matched capacitors.Alternatively, the matched capacitor pair shown may be part of a passivedevice section on the IC chip, a section that may have as many levels ofmetal as are convenient for fabrication of the passive devicestructures, independent of the metal levels in the active deviceregion(s). The same holds true for IPDs, where the number of levels inthe stacked multi-level matched capacitor pair device may be whatever isneeded or convenient.

FIG. 3 shows the matched capacitor pair a-c and b-c connected to otherdevices on the integrated circuit (or to I/O leads in an IPD) at one endof the capacitor pair. To improve high frequency performance it may behelpful to have distributed connections to the matched capacitor pair.FIG. 5 shows the matched capacitor pair of FIG. 3 with interconnections35 to the “back” of the capacitor structure.

The matched capacitor pair structures according to the invention offerconvenient means for adjusting the capacitance ratio of capacitance a-cto capacitance b-c. The above illustrations have had a one-to-one ratioresulting in a matched capacitor pair. The ratio is varied by simplyomitting a portion of the length of one of the interdigitated runners.FIG. 6 shows a capacitor pair similar to that of FIG. 3, but with fiveinterdigitated runners for capacitor a-c, and only three interdigitatedrunners for capacitor b-c. This yields a capacitor ratio a-c to b-c offive to three. Whole finger portions of runners may be eliminated toadjust the ratio, as shown in FIG. 6, or as shown in FIG. 7, fractionalparts of finger portions of runners may be left out, i.e. the fingerportions of runners for one of the plates may simply be made shorterthan the finger portions of runners for the other plate. In theembodiment shown in FIG. 7, where the finger portions of runners forplate b are shorter (in the y-direction) than the finger portions ofrunners for plate a, the capacitance ratio a-c to b-c is the ratio ofthe length of the fingers of runner a overlap with runner c, to thelength of the fingers of runner b overlap with runner c. The resultingcapacitors provide a proportional matched pair of capacitors. Theproportional matched capacitor pairs of FIGS. 6 and 7 may also bedesigned with the plate-interleaved runner spacing shown in FIG. 3 andthe discussion thereof.

As described in connection with FIG. 3, the dimension “p” in FIG. 3should exceed dimension “x” in FIG. 4 in the manner described earlier.In the embodiment of FIG. 6, this allows greater control over the ratioof capacitances between the capacitor pair.

In some circuit applications, the use of a common capacitor plate, i.e.plate 31 of FIG. 3, may be undesirable. A variety of options forsplitting the common plate are available. Two of these are shown inFIGS. 8 and 10 respectively.

FIG. 8 shows a matched capacitor pair with two interleaved intermediateplates, c₁ and c₂. This structure allows the more familiar fourelectrode interconnection for the capacitor pair. The circuit may bedesigned to allow for the small capacitive coupling between c₁ and c₂,or the space separating c₁ and c₂ may be suitably enlarged.

FIG. 9 shows a stacked version of the matched capacitor pair of FIG. 8.Again, five levels are shown, with corresponding plates, and plates thatare vertically aligned are interconnected by vias. As pointed outearlier, the vias are one option, as is the repetition of the patternbetween levels. Other arrangements may be used wherein the capacitanceof the matched capacitor pairs differ between levels or where the plateson adjacent levels differ. Among other options are to place resistorsand/or inductors at some levels. In this manner whole IPD circuits maybe built using two or more levels. Or levels with passive devicecircuits may be used with levels with active devices.

FIG. 10 shows yet another embodiment wherein two interleaved interiorcapacitor plates are provided, thus providing two plates where there isa common interleaved plate 31 of FIG. 3. It will be recognized that theembodiment of FIG. 10 is similar to that of FIG. 8, but the capacitivecoupling between c₁ and c₂ is eliminated by a ground plane “gr”. Theground plane comprises an interleaved runner typically of the samematerial and dimensions as the runners c₁ and c₂. The matched capacitorof FIG. 10 shares the virtues of the embodiment of FIG. 3 in terms ofreduced susceptibility to Δt for example, but with the added featurethat it has a true four-node design, a, b, c₁ and c₂, and the twocapacitors are electrically isolated from one another.

A stacked version of the matched capacitor of FIG. 10 is shown in FIG.11, with the ground plane “gr” isolating the interior plates c₁ and c₂.Although this embodiment is slightly more complex than that of FIG. 4,the electrical performance may be more suitable for some circuitapplications.

The matched capacitor pairs of FIGS. 8 and 10 may also be designed withdifferent capacitances, resulting in a proportional matched pair ofcapacitors, using the techniques shown in FIG. 6 or FIG. 7 and thediscussion thereof, or the plate-interleaved runner spacing shown inFIG. 3 and the discussion thereof.

The conductors that form the capacitor plates are referred to asrunners. It will be recognized that in the capacitor structures of theinvention, the edges or sides of the runners form the capacitorsurfaces. Thus, increasing the width of the runners does not increasecapacitance. Therefore, for space efficiency, the runners should bedesigned with a width the same as any other portion of the metalizationthat serves an interconnection function. That width will typically bethe minimum dimension allowed by the design rule. The thickness of therunners does contribute to the capacitor value. However, an advantage ofthe devices described above is that they may be formed as part of themetalization levels already used to manufacture the IC device. In thatcase the thickness of the runners will have a thickness determined bythe process design. That restriction may not be the case in IPDproduction. Here the runner thickness may be adjusted to suit thecapacitor design. Taking all these factors into account, the aspectratio, width/thickness, for the runners in the devices described herewill most typically be in the range of 3 to 0.5. Other ratios may befound useful. It is noted here that the ratio of width to thickness ofthese runners will also affect the capacitor density, an additionalfactor to be taken into account.

The structures described herein have features that are easily describedheuristically, but a precise prescription of the generic base of theinvention may be aided by additional definitions.

Each of the capacitor plates in the embodiments described comprises amain runner, and a plurality of side runners extending from the mainrunner to form a comb structure. The comb structure is evident in eachof FIGS. 3, 5, 6, 7, 8, and 10. The side runners are analogous to theteeth of the comb, but in the context of an interdigitated arrangement,are referred to here as fingers of the comb.

Among other features of the invention the matched capacitor pairs andproportional matched capacitor pairs described herein comprise at leastthree capacitor plates, with two of the capacitor plates each having acomb structure, and the fingers of one comb structure interdigitatedwith the fingers of the other comb structure. The third capacitor plateextends, or is interleaved, between the two interdigitated plates.According to a common interpretation, the term interdigitated describesfingers from two sources that extend between each other with every otherfinger originating from a first one of the two source and the alternatefingers originating from the second of the two sources. With the outerplates of the capacitor defined above as having a comb structure, thedevices may be described simply as a two-comb structure with the fingersof the combs interdigitated.

The interdigitated fingers and interleaved capacitor plate have aspacing therebetween. Because design rules change, it is preferable toprescribe the spacing as a ratio. Although other spacings may be used, arecommended spacing “s”, as a ratio with the runner width “w”, is:s=0.5 to 3.0 w.

Another feature common to the embodiments shown is that runners thatform at least some of the capacitor plates are boustrophedonic, that is,they follow a meandering or back-and-forth path. These are runners c₁,c₂, and the ground runner gr (FIG. 10). The boustrophedonic path changesdirection in 90° increments, and to form a matched capacitor pair or aproportional matched pair of capacitors in a configuration according tothe invention requires at least two path changes. Thus in the context ofthe invention, a meandering or boustrophedonic path may be defined asone that turns through at least 180°. It is noted that the paths shownin the figures comprise straight sections. Other options may be founduseful.

The various configurations of the invention may alternatively bedescribed by reference to the section views. Thus the capacitor pairshown in FIG. 3 comprises a planar structure having a plurality ofrunners, with a cross section normal to the runners having alternatingrunners in the sequence a, c, b, c, a, . . . wherein runners a, b, c areseparated by insulator, and are, respectively, interconnected.

The process steps for forming the structures described are conventionaland straightforward. The fundamental steps comprise providing asubstrate, the substrate either insulating, or covered with aninsulating layer, depositing a metal layer on the substrate, andpatterning the metal layer to form the metal runners. The metal may bedeposited by any of a variety of known techniques, includingevaporation, sputtering, plating, and chemical vapor deposition (CVD).The deposited metal layer may be patterned by conventionalphotolithography. To take advantage of the process compatibility featureof the invention, all of the metal patterns forming the capacitor platesmay be formed in a single patterning operation. In addition, for ICmanufacture, that single patterning operation forms the metalizationpattern that is used to interconnect other elements of the integratedcircuit on a given metalization level. The term “other elements” ismeant to include resistors, inductors, transistors, diodes, etc.

The matched capacitor pairs or proportional matched capacitor pairsfabricated as described above are fabricated as part of an IPD or a dieand are interconnected to other components on the IPD or if on a die areinterconnected to other components on the wafer. A plurality ofidentical die are typically formed in a repeated pattern on a surface ofthe wafer. The individual die are cut or diced from the wafer, thenpackaged as an integrated circuit. One skilled in the art would know howto dice wafers and package die to produce integrated circuits.Integrated circuits so manufactured including matched capacitor pairs orproportional matched capacitor pairs are considered part of thisinvention.

While the invention has been described mostly with respect to processingsilicon wafers, the invention is not limited thereto. The invention maybe formed using other processing technologies including but not limitedto processing technologies to fabricate matched capacitor pairs orproportional matched capacitor pairs in gallium arsenide and indiumphosphide wafers.

Various additional modifications of this invention will occur to thoseskilled in the art. All deviations from the specific teachings of thisspecification that basically rely on the principles and theirequivalents through which the art has been advanced are properlyconsidered within the scope of the invention as described and claimed.

1. A capacitor pair device including a first capacitor and a second capacitor, comprising: a. a substrate, b. a first conductive comb structure on the substrate forming another plate of the first capacitor of the capacitor pair, c. a second conductive comb structure on the substrate forming a plate of the second capacitor of the capacitor pair, the second conductive comb structure interdigitated with the first conductive comb structure, and d. a continuous conductive runner on the substrate interleaved between the first conductive comb structure and the second conductive comb structure, the continuous conductive runner forming the first capacitor with the first conductive comb structure and the second capacitor with the second conductive comb structure, thus forming the capacitor pair.
 2. The device of claim 1 wherein the capacitor pair is a first level capacitor pair, and the device includes an insulating layer on the capacitor pair, and further includes a second level capacitor pair formed on the insulating layer.
 3. The device of claim 2 wherein the first level capacitor pair is interconnected with the second level capacitor pair.
 4. The device of claim 1 wherein the capacitor pair device comprises an integrated circuit.
 5. A capacitor pair device including a first capacitor and a second capacitor comprising: a. a substrate, b. a first conductive comb structure on the substrate forming a plate of the first capacitor of the capacitor pair, c. a second conductive comb structure on the substrate forming another plate of the second capacitor of the capacitor pair, the second conductive comb structure interdigitated with the first conductive comb structure, d. a first continuous conductive runner on the substrate interleaved between the first conductive comb structure and the second conductive comb structure, the first continuous conductive runner forming the first capacitor with the first comb structure, e. a second continuous conductive runner on the substrate parallel to the first continuous conductive runner, and interleaved between the first continuous conductive runner and the second conductive comb structure, the second continuous conductive runner forming the second capacitor with the second conductive comb structure, thus forming the capacitor pair.
 6. The device of claim 5 wherein the capacitor pair is a first level capacitor pair, and the device includes an insulating layer on the capacitor pair, and further includes a second level capacitor pair formed on the insulating layer.
 7. The device of claim 6 further including a ground plane on the substrate between the first continuous runner and the second continuous runner.
 8. The device of claim 7 wherein the capacitor pair is a first level capacitor pair, and the device includes an insulating layer on the capacitor pair, and further includes a second level capacitor pair formed on the insulating layer.
 9. The device of claim 8 wherein the first level capacitor pair is interconnected with the second level capacitor pair.
 10. The device of claim 7 wherein the first continuous conductive runner, the second continuous conductive runner and the ground plane, each follow a parallel meandering path.
 11. The device of claim 10 wherein the meandering path turns at least 180°.
 12. The device of claim 11 wherein the meandering path includes at least two 90° turns.
 13. The device of claim 7 wherein the fingers of the first and second conductive comb structures, the first and second continuous runners, and the ground plane, all have essentially the same thickness.
 14. The device of claim 13 wherein the fingers of the first and second conductive comb structures, the first and second continuous runners, and the ground plane, all have essentially the same width. 